Interpretive Summary: Disease control and prevention is dependent upon understanding what organisms are causing the diseases. Beginning in 2002 a leaf spot disease of radicchio (Cichorium intybus) was observed in Monterey County, California. Disease incidence in the first year resulted in up to 10% unharvestable radicchio. This research identified the pathogen as a member of the species Xanthomonas hortorum indicating that it may be related to an important bacterial plant pathogen of lettuce. Radicchio is ranked 22 among crops in Monterey County California with a farm gate value of $19,531,000 and is an excellent source of antioxidants. Bacterial diseases of plants limit the supply of high quality food and fiber to consumers. Steps to control this disease will help ensure an adequate supply of this vegetable.

Technical Abstract:
Beginning in 2002 a leaf spot disease of radicchio (Cichorium intybus) was observed in Monterey County, California. Lesions were maroon to dark brown in color; in some cases the margins of brown lesions became dark maroon with aging. Each leaf spot was observable from both adaxial and abaxial leaf surfaces. Bacteria forming yellow mucoid colonies were isolated from surface disinfested symptomatic tissue that was macerated and streaked onto sucrose peptone agar medium and had the same DNA fragment-banding pattern generated by repetitive extragenic palindromic sequence polymerase chain reactions (rep-PCR) using the BOXA1R primer. MLST analysis indicated that radicchio isolates were members of the species X. hortorum. Radicchio plants inoculated with the isolates developed leaf spots similar to those observed in the field. The bacteria isolated from symptomatic tissue on inoculated plants were identical to the original strains when compared with rep-PCR, thus completing Koch’s postulates. To our knowledge this is the first report of X. hortorum causing a leaf spot disease on radicchio.